The style is not to be confused with impromptu preaching, but involves much preparation. One prepares by dwelling on the subject continually and thus has something to say when the time comes.
The first thing to be observed is, that the student who would acquire facility in this art, should bear it constantly in mind, and have regard to it in all his studies and in his whole mode of study. -Henry Ware, Jr.
On the other hand, it is distinct from memorized preaching. Proponents claim that the importance of preaching demands impromptu preaching.
A reflecting mind will feel as if it were infinitely out of place to present in the pulpit to immortal souls, hanging upon the verge of everlasting death, such specimens of learning and rhetoric. -Charles Finney
Extemporaneous preaching includes both preaching with no written preparation as well as brief notes for the purpose of ordering points.
The disadvantages of such a style is the trade-off of precision for emotional emphasis.
This is precisely the manner of the extemporaneous speaker -- in whom the countenance reflects the emotions of the soul, and the tone of voice is tuned to the feelings of the heart, rising and falling with the subject, as in conversation, without the regular and harmonious modulation of the practiced reader.
To preach without premeditation, is altogether unjustifiable; although there is no doubt that a man of habitual readiness of mind, may express himself to great advantage on a subject with which he is familiar, after very little meditation.
Many have preached in this mode for years, and yet have never relaxed their diligence in study, nor declined in the variety, vigor, and interest of their discourses; sometimes dull, undoubtedly; but this may be said with equal truth of the most faithful and laborious writers.